Conventional electrical power systems utilize a synchronous electrical generator for generating AC power. Such a generator may include a rotor and a stator having a stator coil. In applications such as an aircraft, the rotor is driven by an engine such that electrical power is developed in the stator coil. Owing to the variation in engine speed, the frequency of the power developed in the generator winding is similarly variable. This variable frequency power is converted to constant frequency power using a variable speed constant frequency (VSCF) system including a power converter which may develop, for example, 115/200 V.sub.AC power at 400 Hz.
A typical converter includes an AC to DC converter, such as a rectifier, connectable through a DC link having a filter to a DC to AC converter, such as an inverter. The output of the inverter comprises constant frequency power which is applied through a high frequency filter to an AC bus. Typically, the converter is controlled by a pulse width modulation (PWM) control signal to focus energy at the fundamental frequency and to suppress harmonics. The number of harmonics controllable is a function of switching frequency. However, loss is also proportional to switching frequency. Therefore, it is necessary to limit the number of pulses on any given switch. Typically, seven pulses are used, one to control the magnitude at the fundamental frequency and the others to control six harmonics. With a single inverter, this eliminates the third through thirteenth harmonics. A high frequency filter is used to reduce the higher harmonics. However, it is desirable to minimize the filter size.
Higher power requirements cannot always be met with a conventional power system. In certain applications, dual, parallel parts may be used. For example, dual inverters controlled identically may be connected in parallel. Ideally, the dual inverters are controlled so that loading is split equally between the two. For connection of the same in parallel, the corresponding phase outputs of each may be connected through an interphase transformer. With an interphase transformer, if the DC component on each inverter output is unequal, then a DC current flows through the interphase transformer. The amount of magnetic flux swing changes as a result of this current. Therefore, it is desirable to eliminate the DC current flowing through the interphase transformer.
With respect to converter output, aircraft specifications generally require a reference. A neutral forming transformer comprises a load of star configuration in hard parallel with the output to form a neutral.
Any non-ideal characteristics of any of the inverter, the filter or the load produces DC offsets within the converter system. These offsets cause DC current to flow, with the current amount being related to DC voltage levels. The magnetic design of the interphase transformers and the neutral forming transformer are a function of DC content in the inverter output. This results in a heavier, more inefficient and larger design, all of which are undesirable in aircraft applications.
The present invention is intended to overcome one or more of the problems as set forth above.